Particularly in the case of phase masks as lithography masks for circuits having a high integration density, the bend locations of convex structures have an unstable behavior with regard to a defocusing of the exposure system used. A convex structure is understood to be an angled structure element formed by a first and a second segment opaque to the exposure radiation where the structure element has a reflex angle. The region facing the reflex angle is referred to as convex section with the convex bend location, and the region facing away from the reflex angle is referred to as concave section of the angled structure element.
FIGS. 1a and 1b diagrammatically illustrate specifically an alternating bright-field phase mask as a lithography mask having in each case an angled structure element O of the type known from the prior art.
The two opaque segments O1 and O2 formed such that they are of the same size and rectangular, in each case form an angled structure element O with a reflex angle α=270° in the so-called convex section A of the angled structure element O. Located opposite the reflex angle α in each case is a right-angled counterpart angle α′=90° in the so-called concave section A′ of the angled structure element O. On the part of the convex section A, a first segment T that is transparent with regard to the exposure radiation extends adjacent to the two opaque segments O1 and O2.
In this patent application, the feature adjacent is understood to be a direct adjoining of the adjacent regions without an interspace or intermediate section.
On the part of the concave region A′, a second segment T′ that is transparent with regard to the exposure radiation extends adjacent to the two opaque segments O1 and O2. Both transparent segments T and T′ have an angled form corresponding to the angled structure element O.
On the side facing away from the angled structure element O, the transparent segments T and T′ illustrated in FIGS. 1a and 1b are in each case surrounded by regions of the phase mask which are opaque to the exposure radiation. Like the opaque segments O1 and O2, these regions are usually formed as thin metal layers, for example made of chromium.
The second transparent segment T′ is formed such that it is in antiphase with respect to the first transparent segment T. In antiphase is to be understood such that the exposure radiation of the phase mask has experienced a mutual phase shift of 180° after passing through the transparent segments T and T′. This in-antiphase formation of different transparent segments on a phase mask is also illustrated by the designation “alternating” phase mask.
However, the unstable behavior at convex structures occurs not only in the case of alternating phase masks but also in the case of conventional lithography masks without phase-shifting regions.
For graphic illustration, the transparent segments are depicted as either hatched or checked in all the figures, there being a phase difference of 180° in each case between the hatched and checked segments.
For reasons of better clarity of the graphic illustration, both the reflex angle α and the associated counterpart angle α′ are in each case not depicted directly at the convex and concave bend location of the angled structure element O, but rather outside the hatched and checked regions, respectively.
The destructive interference effect between two closely adjacent and coherent light beams having phases shifted through 180° can be exploited in order to be able to expose structures that are narrower than the wavelength of the exposure radiation used. No photosensitive material is exposed in the region in which the light beams interact with one another. The simulation calculations for the exposure of convex structures explained below were carried out in FIGS. 1a and 1b for an exposure radiation having the wavelength λ=193 nm with a partial coherence factor for the phase mask of σ=0.35, the angled structure element O having a width of 150 nm in FIG. 1a and of 100 nm in FIG. 1b. The convex bend locations having the angle α=270° of the angled structure elements O are so small in relation to the wavelength of the exposure radiation that their images appear as rounded structures even with ideal focusing. This is illustrated in FIGS. 1a and 1b on the basis of the rounded solid lines in the bend regions of the angled structure elements.
In the concave bend region, the rounding is due to the fact that the intensity of the exposure radiation in this region, on account of the small dimensions of the bend region in comparison with the wavelength of the exposure radiation, does not suffice to ensure an exposure right into the rectangular concave corner region.
In the convex bend region, by contrast, constructive interferences of the in-phase exposure radiation from regions of the first transparent segment T that are adjacent to the two opaque segments O1 and O2 are the cause of the instances of rounding that occur.
The extent of rounding essentially depends on the focusing. With non-ideal focusing (the rounded solid line shows the result of a simulation calculation for the defocus value 0.0 μm), the rounding that occurs in the convex bend region, with increasing defocusing advances ever further in the direction of the concave bend region (dash-dotted line for defocus value 0.3 μm) and finally even changes the direction of curvature into the direction of the concave bend region (dotted line for the defocus value 0.4 μm).
This effect is referred to as unstable behavior of the convex structure with regard to the defocusing and is a consequence of severe constructive interferences. Comparison of FIGS. 1a (150 nm width of the opaque segments) and 1b (100 nm width of the opaque segments) reveals: the narrower the structure to be exposed or the higher the defocus value, the more serious are the undesirable degenerations of the convex bend regions.
By contrast, the degree of defocusing of the exposure radiation does not affect the position and formation of the rounding in the concave bend region. The solid, dash-dotted and dotted lines run essentially congruently here.